The present invention relates to a scanning optical device such as a laser beam printer. Particularly, the present invention relates to a multi-beam scanning optical device that forms a plurality of scanning lines per scan.
The multi-beam scanning optical device deflects beams by a polygonal mirror emitted from light sources such as laser diodes, and converges the beam to form a plurality of spots on a scanned surface such as a surface of a photoconductive drum, through an f.theta. lens (scanning lens). The beam spots formed on the scanned surface move (i.e., scan) on the scanned surface in a predetermined scanning direction as the polygonal mirror rotates. The scanning optical device is provided with a beam detecting sensor that detects scanning timings of respective scanning beams at upstream of a drawing area. A controller starts to modulate each of the beams per scan after predetermined time interval when the beam detecting sensor detects the scanning timing. The beams are independently modulated according to drawing data such that the beam spots form a plurality of scanning lines per scan.
In this specification, the direction in which the beam spot scans on the scanned surface is referred to as a main scanning direction. Further, a direction perpendicular to the main scanning direction on the image plane is referred to as an auxiliary scanning direction. The orientations of the shape and the refractive power of optical elements are defined with reference to the directions on the image plane.
In a scanning optical system employed in the multi-beam scanning optical device, drawing start positions, from which the scanning beam spots contribute to image formation, and drawing complete positions, which are the ends of the image portion on the scanning line, should be coincident with one another to keep drawing accuracy.
In the multi-beam scanning optical device, generally, wavelengths of the beams emitted by the plurality of laser diodes distribute. Namely, there is variability in an emission wavelength of a laser diode. Additionally, the f.theta. lens is not compensated in a chromatic aberration in general, because the f.theta. lens is used for the predetermined design wavelength. Therefore, the distribution of the wavelength of the beams changes the width of the drawing area between a plurality of lines. Furthermore, the chromatic aberration of the f.theta. lens changes the detected timing by the beam detecting sensor when the wavelength is different from the design wavelength.
When the wavelength is larger than the design wavelength, the refractive power of the f.theta. lens decreases, which enlarges the width of the drawing area and delays the detected timing by the beam detecting sensor. Alternatively, when the wavelength is smaller than the design wavelength, the refractive power of the f.theta. lens increases, which reduces the width of the drawing area and moves up the detected timing by the beam detecting sensor.
As a result, the variation of the wavelength of the beams shifts the drawing area between a plurality of lines, which deteriorates the quality of formed image.
FIG. 5 is a timing chart showing the drawing start and complete timings for two beam spots formed by beams having different wavelength. The timing charts is described on the assumption that two beams are simultaneously deflected by the polygonal mirror to form adjacent scanning lines on the scanned surface and two beam spots scan the same position in the main scanning direction when the wavelength of the two beams are identical.
In FIG. 5, a first beam spot B.sub.1 formed by a beam in the design wavelength .lambda..sub.0 passes the beam detecting sensor at timing t.sub.1 and then after a predetermined time interval .DELTA.t.sub.1, the first beam spot B.sub.1 reaches the drawing start timing t.sub.3. After a predetermined time interval .DELTA.t.sub.2 from the timing t.sub.3, the first beam spot B.sub.1 reaches the drawing complete timing t.sub.5.
A second beam spot B.sub.2 formed by the beam in the wavelength .lambda..sub.1 (.lambda..sub.0 &lt;.lambda..sub.1) passes the beam detecting sensor at timing t.sub.2 with a little delay from the timing t.sub.1. After the predetermined time interval .DELTA.t.sub.1, the second beam spot B.sub.2 reaches the drawing start timing t.sub.4. After a predetermined time interval .DELTA.t.sub.2 from the timing t.sub.4, the second beam spot B.sub.2 reaches the drawing complete timing t.sub.5.
The time intervals .DELTA.t.sub.1 and .DELTA.t.sub.2 are identical for the two beam spots B.sub.1 and B.sub.2, only the timings of the second beam spots B.sub.2 are delayed from the timings of the first beam spot B.sub.1. The timing shift described in FIG. 5 appears as the shift of the drawing areas in consideration of the width variation of the drawing area.
FIG. 6 is a chart showing positions of the beam spots B.sub.1 and B.sub.2. The first beam spot B.sub.1 starts the drawing at the intended drawing start position P.sub.1 and then finishes at the intended drawing complete position P.sub.2. On the other hand, the second beam B.sub.2 starts the drawing at a position P.sub.4 that is shifted downstream from the intended drawing start position P.sub.1 and then finishes at a position P.sub.5 that is largely shifted downstream from the intended drawing complete position P.sub.2. The drawing area of the first beam spot B.sub.1 is an area between the drawing start position P.sub.1 and the drawing complete position P.sub.2, that for the second beam spot B.sub.2 is defined as an area between the positions P.sub.4 and P.sub.5. References P.sub.3 and P.sub.6 show the centers of the drawing areas for the first and second beam spots, respectively.
The drawing start timing t.sub.4 of the second beam spot B.sub.2 is delayed from the drawing start timing t.sub.3 of the first beam spot B.sub.1, which shifts the drawing start position of the second beam spot B.sub.2 in the downstream direction with respect to that of the first beam spot B.sub.1, because the deflection angle formed by the polygonal mirror varies. On the contrary, the lateral chromatic aberration of the f.theta. lens enlarges the drawing area for the second beam spot B.sub.2 as compared with that of the first beam spot B.sub.1, which shifts the drawing start position of the second beam spot B.sub.2 in the upstream direction with respect to that of the first beam spot B.sub.1. As a result, the drawing start position P.sub.4 of the second beam spot B.sub.2 is slightly shifted downstream with respect to the intended drawing start position P.sub.1.
At the drawing complete position, since both of the delay of the timing and the shift due to the literal chromatic aberration shifts the drawing complete position of the second beam spot B.sub.2 in the downstream direction with respect to that of the first beam spot B.sub.1, the drawing complete position P.sub.5 is largely shifted downstream with respect to the intended drawing complete position P.sub.2.
Thus the drawing area of the second beam spot B.sub.2 is shifted downstream with respect to the intended drawing area. Particularly, the deviation of the beam spot at the side of the drawing complete position becomes significant, which deteriorates the quality of formed image.
A use of an achromatic f.theta. lens solves the above described problem. However, since the achromatic f.theta. lens requires a combination of lens elements having different dispersion, the number of lens elements of the f.theta. lens increases when compared with a case where the chromatic aberration is not corrected, which increases the cost of the device.